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The vaccine has an 18 percent effectiveness against the dominant H3N2 flu strain, the CDC reports. Earlier in the season, flu shot protection level was estimated at 23 percent.

"There's more data that's come in now, given how much of the flu is caused by this mutated strain of H3N2," ABC News' Chief Health and Medical Editor Dr. Richard Besser told ABC News today. "It turns out the vaccine is even less effective than they thought."